Abstract
Whereas, there is data to support that cuneothalamic projections predominantly reach a topographically confined volume of the rat thalamus, the ventroposterior lateral (VPL) nucleus, recent findings show that cortical neurons that process tactile inputs are widely distributed across the neocortex. Since cortical neurons project back to the thalamus, the latter observation would suggest that thalamic neurons could contain information about tactile inputs, in principle regardless of where in the thalamus they are located. Here we use a previously introduced electrotactile interface for producing sets of highly reproducible tactile afferent spatiotemporal activation patterns from the tip of digit 2 and record neurons throughout widespread parts of the thalamus of the anesthetized rat. We find that a majority of thalamic neurons, regardless of location, respond to single pulse tactile inputs and generate spike responses to such tactile stimulation patterns that can be used to identify which of the inputs that was provided, at above-chance decoding performance levels. Thalamic neurons with short response latency times, compatible with a direct tactile afferent input via the cuneate nucleus, were typically among the best decoders. Thalamic neurons with longer response latency times as a rule were also found to be able to decode the digit 2 inputs, though typically at a lower decoding performance than the thalamic neurons with presumed direct cuneate inputs. These findings provide support for that tactile information arising from any specific skin area is widely available in the thalamocortical circuitry.
Highlights
Information about tactile inputs is ubiquitous in the rat cortical circuitry, including primary visual cortex as demonstrated for a set of tactile input patterns delivered to digit 2 (Enander et al, 2019)
We made extracellular unitary spike recordings (Figure 1A) from 109 thalamic neurons across the rostrocaudal and mediolateral extent of the thalamus (Figure 2). We analyzed these neurons with respect to their spike shapes, spike firing patterns, response latency times and decoding of a set of eight spatiotemporal tactile afferent activation patterns delivered to the ventral side of the distal digit 2
We found that putative thalamic projection neurons distributed throughout all parts of the thalamus that we recorded from could decode tactile input patterns delivered to distal digit 2
Summary
Information about tactile inputs is ubiquitous in the rat cortical circuitry, including primary visual cortex as demonstrated for a set of tactile input patterns delivered to digit 2 (Enander et al, 2019). As the cortex projects back to the thalamus, and not just to homonymous thalamic nuclei (Halassa and Sherman, 2019), but to diverse parts of the thalamus, it would seem a natural consequence that in the thalamus there would be ubiquitous representation of tactile information. This possibility has not been explored in great detail, as the focus in thalamic recording studies typically lie in the study of the modality expected to be specific for the thalamic nuclei recorded from. Bieler et al (2018) found tactile inputs to evoke field potential responses in the lateral geniculate nucleus (LGN) They found that individual cells in the VPM can project directly to Tactile Decoding in Thalamic Neurons the primary visual cortex. These findings show that tactile inputs can reach wide cortical areas, and since these cortical areas project back to different primary thalamic regions, a wide region of the thalamus which may not receive direct cuneate nucleus input (Alloway et al, 1994; Bermejo et al, 2003) can potentially be reached by tactile inputs via longer pathways
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